1. Field of the Invention
The present invention relates to a fuel cutoff valve mounted on an upper wall of a fuel tank to open and close a connection conduit, which connects the inside of the fuel tank with the outside.
2. Description of the Related Art
FIG. 5 shows a prior art fuel cutoff valve 100 having a known structure disclosed in U.S. Pat. No. 5,404,907. The fuel cutoff valve 100 is mounted on an upper tank wall FTa of a fuel tank FT, and includes a casing 102, a cover 110, a float 120, and a spring (not shown). The casing 102 has an upper wall 103, a side wall 104 that extends from the outer circumference of the upper wall 103 and is integrally molded with the upper wall 103, and a bottom plate 105 that is attached to a lower end of the side wall 104. An inner space of the casing 102 forms a valve chest 1028.
The float 120 having a valve element 120a disposed on an upper wall thereof is accommodated in the valve chest 102S. The valve element 120a opens and closes a connection conduit 103d that connects with inside of the fuel tank FT with outside. The float 120 is supported by the spring (not shown) mounted on the bottom plate 105.
The cover 110 includes a cover main body 112 fixed to the casing 102, a cover passage forming element 114, and a flange 115. These constituents of the cover 110 are integrally molded. The cover main body 112 has an attachment recess 114a, in which an upper projection element 103b of the upper wall 103 of the casing 102 is fitted. Engagement apertures 103a are formed in the upper wall of the casing 102 to receive engagement projections 112a formed on the cover 110 and fitted therein, so that the casing 102 is joined with the cover 110. A join end 115a of the flange 115 is thermally welded to the upper tank wall FTa of the fuel tank FT.
In the fuel cutoff valve 100 of the above structure, in the course of fuel supply into the fuel tank FT, the fuel vapor in the fuel tank FT flows out to a canister (not shown) via a through hole 105a formed in the bottom plate 105 of the casing 102, the valve chest 102S, the connection conduit 103d, and a passage 114b of the cover 110. When the level of the liquid fuel in the fuel tank FT reaches a particular level FLI, the liquid fuel flows into the valve chest 102S via the through hole 105a and gives the buoyancy to raise the float 120. The rise of the float 120 causes the valve element 120a formed on the upper wall of the float 120 to close the connection conduit 103d, so as to prevent the fuel from flowing out of the fuel tank FT.
In the prior art fuel cutoff valve 100, however, the joint of the casing 102 with the cover 110 is attained only through the engagement of the engagement projections 112a with the engagement apertures 103a and thereby readily becomes unsteady to make the casing 102 movable to the cover 110. The cover 110 is made of polyethylene and is thermally welded to the fuel tank FT, which is also made of polyethylene, whereas the casing 102 is made of polyacetal having a smaller fuel swelling property than polyethylene, in order to ensure the high sealing properties. Namely the cover 110 and the casing 102 are made of different resin materials. When the casing 102 and the cover 110 swell in the liquid fuel in the fuel tank FT, the difference in swelling properties between the cover 110 and the casing 102, which are joined with each other only through the engagement of the engagement apertures 103a with the engagement projections 112a, may cause the casing 102 to be movable against the cover 110. Such unsteady joint undesirably lowers the mechanical strength of the fuel cutoff valve 100 against shocks and varies the opening and closing liquid levels of the fuel cutoff valve 100. These lead to the unstable valve opening and closing properties.